The invention relates to a luminaire with a multicolored array of light emitting diodes (LEDS). More particularly, the invention relates to a white light emitting luminaire where the light output and the color of the white light produced by the luminaire vary with temperature.
U.S. Pat. No. 5,851,063, assigned to General Electric Company, herein incorporated by reference, describes a system of at least three multi-colored LED""s having an optimized color rendering index by proper selection of the wavelengths of each LED. The reference also includes a method which defines the chromaticity of an LED system by specifying the desired chromaticity of the system, calculating the theoretical x, y, and z CIE (Commission Internationale de L""Eclairage) coordinates of the desired chromaticity on a blackbody locus, and providing light from each LED at the selected wavelength based on those coordinates.
However, under actual conditions, light output for LEDs varies with temperature of the LEDs. This variation does not occur uniformly for each color. In a block of LEDs of a given color, the light output will vary, for example, if one or more of the LEDs fails. Given the factors which can affect the light output and color temperature of any array of LEDs, it would be desirable to automatically control light output and color temperature, especially in a white-light emitting luminaire.
It would also be desirable to control the color and light output of a white light emitting luminaire automatically, continuously, on-line, and with small sized arrays.
According to the invention, an array of LEDs, made up of at least one LED in each of a plurality of colors in a luminaire, is driven by an experimental method. First, electrical current is supplied to the LEDs in each color, so that they have a light output with a nominal continuous value during ordinary operation. Then CIE xy coordinates for each LED light source are measured for different temperatures. The CIE xy coordinates for the LED light sources are expressed as a function of temperature of the LED light source and the expressions are stored in memory. Equations are derived for the CIE x and CIE y coordinates as a function of temperature and are used to calculate the CIE xy coordinates and lumen output fractions while on line. Light output and color of the LEDs in controlled based upon the calculated xy coordinates and lumen output fraction.
In this approach, the CIE xy coordinates for the LED light sources are first measured for different temperatures experimentally for the entire operating range. Then, based on the experimental data, the equations for the CIE x and CIE y coordinates are derived as a function of temperature using polynomials. If the CIE xy coordinates are highly non-linear, then higher order polynomials are used to derive the equations.
Since the CIE xy coordinates are expressed as a function of temperature, the reference lumens are calculated on-line. Therefore, continuous control is provided for the color control. This method is well suited for variable color and lumen control. Measured light outputs are feed-back to a controller along with the desired outputs, which may be set by user controls, and changes to the power supply for color blocks are made as necessary. Color of the white light is thus automatically controlled without regard to factors that may cause it to change. The user inputs permit varying the desired color of the white light to either warm white (more red output) or cool white (more blue output).
In accordance with an embodiment of the invention, for a fixed color of the white light, the reference lumens are calculated off-line first and are expressed as a function of temperature by using polynomials. The temperature is inputted and these equations are then used to obtain the reference lumens on-line.
A further aspect of the invention provides a method to vary the color temperature of the white light linearly. An example of this variance is shifting the color temperature from warm-white to daylight-white. This is achieved by expressing the CIE xy coordinates of the white light on the blackbody locus as a function of color temperature using polynomials. Knowing the CIE xy coordinates of the white light for the desired color point and the CIE xy coordinates of the LED light sources Ad depending on the temperature, the required reference lumen output for each of the colored LED light sources are calculated. These lumens outputs are supplied to a lumen output control system as reference which regulates the lumen output of the LED light sources regardless of temperature and aging.